This invention relates to a method of detecting velocity or displacement of a moving object.
Detecting of the velocity of a moving object and primarily a slow moving vehicle such as an agricultural or other ground working vehicle can be carried out using many techniques.
Physical contact with the ground by a wheel or the like has the disadvantage that slip caused by ground conditions can raise significant inaccuracies.
Radar systems use emissions which are problematic and are ineffective at low speeds close to stationary so that it is not possible to discern very small movements and to distinguish those from no movement.
GPS systems are insufficiently accurate to determine small movements and also signals can be lost at particular locations due to interfering objects.
It is one object of the present invention therefore to provide an improved method for detecting velocity and/or displacement of an object relative to a surface.
According to one aspect of the invention there is provided a method for detecting relative displacement between an object and an illuminated surface comprising:
providing an array of detecting elements each arranged to receive light from a portion of a field of view and to provide an output responsive thereto;
locating the array adjacent the surface during said displacement such that the field of view is moved across the surface by the displacement;
repeatedly acquiring the output signals of the detecting elements to generate a plurality of sequential signals each corresponding to a respective image of the surface with each successive image being displaced from the previous image by a distance proportional to an amount of said displacement;
selecting one of the sequential signals as a reference signal;
processing the sequential signals to effect a comparison of said reference signal with at least two subsequent sequential signals to generate for each subsequent sequential signal a value indicative of displacement between the image corresponding to the reference signal and the subsequent images corresponding to said at least two subsequent sequential signals;
and analyzing the values to determine a final value indicative of the displacement.
It will be appreciated that the output from the method may be a displacement, that is a particular distance moved, or the same value over a particular time period may be used to calculate an instantaneous value of velocity.
The method defined above may be used to detect movement of the surface or the sensor and in many uses, it does not matter which is moving relative to the other. However one primary use of the method is in detection of movement of slow moving vehicles where the surface is the ground and the sensor elements are mounted at a suitable location on the vehicle.
Preferably the reference signal is maintained for said comparison of each image in turn for as long as possible until a predetermined parameter is reached which indicates that the reference frame should no longer be maintained.
In one arrangement, the reference signal is maintained until a meaningful comparison can no longer be made.
In one preferred arrangement, the method includes calculating for each comparison a Q factor indicative of the strength of the displacement value relative to other values wherein the reference signal is maintained for said comparison until the Q factor falls below a predetermined minimum value.
In this arrangement, the reference signal may be changed after a predetermined time period even if the Q factor remains above the predetermined minimum value, to realign the device with subtle changes in the reference signal.
Also the reference signal can be changed after a predetermined displacement even if the Q factor remains above the predetermined minimum value. This will provide more accuracy when there are many symmetries in the reference signal, keeping the Q factor artificially high.
In most cases, the reference signal is changed to that subsequent sequential signal which is being compared when the predetermined parameter, such as the Q factor reaching the minimum acceptable value, is reached.
In a preferred arrangement, the step of analyzing the values includes discarding one or more of the values corresponding to at least one of the subsequent sequential signals. This is done by detecting those values which are physically impossible or statistically improbable based upon data up to date. In such a case, the signal is discarded and the comparisons continued on the basis of the next signal.
While ambient light can be used in some cases, it is preferred to effect separate illumination of the surface to maintain a constant level of illumination. Infrared light is preferred to avoid the variations in ambient illumination which can occur in outside measurement situations.
Preferably the array is two dimensional for detecting displacement in two directions but if movement is limited to a single direction, the array can be arranged along a single line.
Preferably the step of processing includes calculating the convolution integral of the signals, which is a known technique for comparing two digital images and for calculating the displacement therebetween.
While other calculation techniques are available, one preferred arrangement uses a Fast Fourier Transform technique to calculate the convolution integral.
Where the method is used for detecting movement of a slow moving vehicle such as an earth working vehicle, the array is mounted on the vehicle behind one wheel so as to obtain images of a portion of the ground which has been compressed by the wheel. This reduces changes in the image caused by moving vegetation or other objects since those objects are likely rendered stationary by the compression.